The rapid increase in technology, particularly technology directed toward consumer use and the emphasis on portability and light weight in consumer electronic equipment has increased the demand for reliable, light weight, high energy power sources. Such demand is found in a variety of technologies such as power tools, calculators, computers, cordless telephones, garden tools, as well as back-up power sources in computer technology and memory devices.
Lithium nonaqueous cells have long been known and attractive commercially for a variety of reasons including their known high energy density and long shelf-life. Primary lithium nonaqueous cells are used commercially in a variety of applications where high energy density and long shelf life are at a premium. Typical applications are power sources for watches, calculators, pacemakers, rockets, etc.
Particularly desirable for a variety of applications is a secondary lithium nonaqueous battery which retains the advantages of primary lithium cells but can be cycled a large number of times. The range of applications of such a rechargeable lithium nonaqueous cell is enormous and includes, for example, power sources for cordless telephones, portable computers, portable garden tools, portable drills, etc.
Much progress has been made in devising a suitable rechargeable lithium nonaqueous cell. For example, a number of cathode materials have been discovered which have excellent capacities, have good recycling properties and are very compatible with lithium anode material. Particularly noteworthy among these cathode materials are the transition-metal chalcogenides (e.g., NbSe.sub.3); see, for example, U.S. Pat. 3,864,167 issued to J. Broadhead et al. on Feb. 4, 1975.
A nonaqueous cell is generally made up of positive and negative electrodes separated by an insulating material called a separator which prevents electrical contact between positive and negative electrodes, but allows ionic conduction between these electrodes. The separator is usually made of a polymer material (e.g., polyethylene, polypropylene, etc.) made in the form of a microporous film. Typical commercial products are Celgard.RTM.2400 and Celgard.RTM.2402 made by the Celanese Corporation.
Separator materials play an important part in lithium rechargeable battery operation. The separator material must be stable to the conditions of the battery so as not to degrade and allow direct electrical contact between positive and negative electrodes. This is particularly difficult in a lithium nonaqueous battery because of the high reactivity of the materials involved and the high electropotentials involved. The separator material should also be highly insulating to prevent current leakage between positive and negative electrodes and remain highly insulating throughout the life of the battery.
In addition, the separator material must permit high ionic conductivity between positive and negative electrodes so that the cell exhibits high charge and discharge rates. Such high ionic conductivity requires that the separator material be "wetted" by the electrolyte system, but electrolyte systems for lithium nonaqueous batteries are typically highly inert chemically and do not easily "wet" the separator material. Typically, a wetting agent in the form of a surfactant is added to the separator, such as silicon glycol or imidazole, but such agents react with various materials in the cell (probably lithium) and limit the cycle life of the battery. Another solution is to incorporate various substances with low surface tension in the electrolyte (e.g., furans, other ethers, etc.), but these substances also are not as stable as might be desired and might limit the cycle life of the battery.
It is highly desirable to wet the separator material without adding undesirable material to the electrolyte system. Under these circumstances, excellent, inert, long lasting electrolyte systems can be used without degrading charge and discharge current rates.
A number of references have described electrolyte systems for lithium, nonaqueous batteries. (See, for example, a paper by S. I. Tobishima et al., entitled "Ethylene Carbonate/Ether Mixed Solvents Electrolyte for Lithium Batteries", published in Electrochimica Acta 29, No. 10, pp. 1471-1476 (1984) and S. I. Tobishima et al., "Ethylene Carbonate-Propylene Carbonate Mixed Electrolytes for Lithium Batteries", Electrochimica Acta 29, No. 2, pp. 267-271 (1984).) Also of interest is a paper by G. Pistoia entitled "Nonaqueous Batteries with LiClO.sub.4 -Ethylene Carbonate as Electrolyte, Journal of the Electrochemical Society 118, No. 1, pp. 153-158 (1971).
Gamma-ray grafting is a well known technique in polymer chemistry. Indeed, it is used extensively in the production of a variety of articles and is the subject matter of various chapters and books (See, for example, Radiation Chemistry of Polymeric Systems, by A. Chapiro, Interscience, New York, 1962, especially chapter XII, and Atomic Radiation and Polymers, by A. Charlesby, Pergamon Press, New York, 1960).
In aqueous batteries such as alkaline zinc batteries, various grafted separators have been used to promote separator wettability and ionic conduction. Typical references are as follows: "Modifying Membrane Properties to Meet Industrial Needs" by V. D'Agostino et al, Extended Abstracts, 158th Meeting of the Electrochemical Society, Hollywood, Fla., Oct. 5-10, 1980, pp. 1535-1536; a paper by R. S. Yeo and J. Lee entitled "Novel Hybrid Separators for Alkaline Zinc Batteries", Proceedings of the Symposium on Advances in Battery Materials and Processes, Ed. by J. McBreen, D. T. Chin, R. S. Yeo and A. C. C. Tseung, The Electrochemcial Society Proceedings, Volume 84-4, pp. 206-217; and V. D'Agostino et al, "Manufacturing Methods for High Performance Grafted Polyethylene Battery Separators", AFML-TR-72-13, May 1972.